Lead-acid storage batteries comprise several galvanic cell elements each encased in separate compartments of a substantially leak-proof container containing sulfuric acid electrolyte. Each cell element typically comprises at least one plate-like, positive electrode (i.e., positive plate), one plate-like negative electrode (i.e., negative plate) and a porous separator (e.g., a thin microporous sheet and/or absorbent glass mat) therebetween. Multi-plate cell elements are commonplace and comprise a stack of alternating positive and negative polarity plates interleaved one with the other and the separators. The plates themselves each comprise a conductive substrate which supports an electrochemically active material thereon and conducts electrical current substantially uniformly therethrough. In Pb-acid batteries, the plates comprise a leady active material (i.e., PbO.sub.2 for the positive plates and Pb for the negative plates) pasted onto a reticulated Pb-alloy (e.g., Pb-Ca-Sn or Pb-Sb) grid substrate. A lug projects from each grid and serves to electrically couple its associated plate to other electrical components of the battery. For example, aligned lugs of like polarity plates of a multi-plate cell element are commonly electrically coupled one to the other and to intercell connectors or terminals by a so-called plate strap which is typically burned to, or cast about, the plates' lugs. Heretofore, essentially three techniques have been proposed to make such plate straps. In the first technique, a pre-cast plate strap having a plurality of toes, interdigitated with the plate lugs, is burned to the plate lugs by means of a gas torch applied directly thereto. This technique has been used commercially for many years. In the second technique, upstanding plate lugs are enclosed in a mold and molten lead poured into the mold to form the plate strap. A third technique is a variation of the second wherein an open-topped mold is first filled with a predetermined amount of molten lead and thereafter the plate lugs of an inverted cell element are immersed therein. The solidified strap is subsequently removed from the mold, the cell element returned to its upright position and finally inserted into a battery container. Practice of both the second and third so-called "cast-on-strap" techniques were performed essentially in the open air with some attempts being made to flood at least the mold area with inert gas in an effort to exclude air therefrom and thereby reduce the formation of oxides.
The aforesaid second technique never achieved widespread, if any, commercial use presumably owing to the inability to reliably implement such a process on a production scale. In this regard, metal which was poured into the mold at a single site was expected to spread uniformly throughout the mold (i.e., in and around the upstanding plate lugs) and still bond well to the lugs. However, non-uniform distribution of the lead in the mold as well as non-uniform cooling of the melt usually occurred and resulted in poor quality, high resistance connections between the plate lugs and plate strap. The aforesaid third technique substantially eliminated the non-uniform flow, cooling and heat distribution problems of the second technique and has been used commercially for many years. However, it too has disadvantages. In this regard, commercial practice of this technique requires the use of a melting/holding furnace containing a large supply of molten lead ready for pouring, as well as an associated plumbing network of melt delivery pipes, valves, nozzles, etc. This complicated assemblage of melt handling equipment is cumbersome, requires considerable maintenance and has to be kept hot at all times even when plate straps are not being cast. The heat required to melt and keep large quantities of lead molten, as well as keep the melts' delivery plumbing hot, not only results in a costly consumption of energy but radiates into the work area making it a less desirable operator working environment. Moreover, the maximum practical temperature useful with such prior "cast-on-strap" techniques was effectively limited to about 850.degree. F. above which untoward oxidation of the molten lead occurs which tends to cause equipment fouling and result in oxide inclusions in the casting. As a result, in actual practice, melt is typically cast from such machines at temperatures below about 830.degree. F. Such a low lead temperature reduces the ability of the molten lead to effectively melt the solid lugs in the mold and thereby results in unnecessarily high electrical resistance between the lugs and the plate strap. For example, and as shown in FIG. 8 hereof, such processes resulted in melting only the tips of the lugs with the melt cast into the mold. The remainder of the lug is left substantially intact with only the surfaces thereof bonded to the solidified plate strap. Finally, experience has shown that the equipment commercially available for metering and delivering the lead into the molds by those techniques is not capable of consistently casting plate straps of the exact same size. Rather, the mass of the plate straps varies significantly one from another over the course of a production run. As a result, in order to insure that each and every plate strap has at least the minimum amount of lead necessary for current conduction and strength it is common practice to purposely set the delivery equipment to dispense more lead (often as much as 30% more) into the molds than is theoretically necessary. Hence many straps are cast with more lead than needed which results not only in heavier batteries but considerable excess cost.
Other lead battery parts are also welded together in a battery. For example, intercell connectors and connections made between plate straps and sidewall terminals have both been made both by gas torch and electrical resistance welding. Moreover, the GTAW process, often referred to as TIG welding, has been used to make intercell connections as disclosed in Johnson et al 4,177,551. However, Johnson et al's direct TIG welding technique is susceptible to oxidation of the molten metal while at the arc-melting temperature and can cause splattering of the lead which not only leaves an unsightly joint but increases the possibility of lead splatter falling into and contaminating the cell elements in the vicinity of the weld. Such direct TIG welding is not acceptable for forming plate straps to lugs. In this regard, trying to adapt such a process to the joining of a plurality of disconnected plate lugs would require either additional tooling or a solid bar of lead in the trough in order to provide an arc-sustaining counterelectrode therein. Finally, it has been found that in the absence of a bar of lead in the trough along with the lugs, directly applied TIG arcs: (1) completely destroy lead-calcium-tin alloy lugs; and (2) ignite low melting plastic trough materials such as, for example, are disclosed in copending Bish et al U.S. patent application Ser. No. 859,364 filed May 5, 1986, now U.S. Pat. No. 4,683,180 and assigned to the assignee of the present invention.
Accordingly, it is an object of the present invention to provide a unique, simple, energy-efficient process and apparatus for joining lead battery parts together via a low resistance, substantially oxide-free connection cast therebetween. It is another object of the present invention to provide a unique, simple, energy-efficient process and apparatus for casting a substantially oxide-free plate strap to a set of aligned, upstanding, lead-acid storage battery plate lugs wherein controlled amounts of substantially oxide-free, superheated, molten lead is cast about the lugs in such a manner as to cause the lugs to substantially completely amalgamate with the lead cast thereabout. It is a further object of the present invention to provide a unique, simple, energy-efficient process and apparatus for assembling a Pb-acid storage battery wherein substantially oxide-free, low resistance plate straps and an intermediate intercell connector are formed in a single casting operation within the battery container so as to amalgamate with battery plate lugs associated with the straps. These and other objects and advantages of the present invention will become more readily apparent from the description thereof which follows.